Titanium, including the many known grades of commercially pure titanium and alloys of titanium (wherein titanium comprises the major constituent), possesses very desirable corrosion resistance in a wide variety of environments. For example, both commercially pure titaniums and alloys of titanium have demonstrated good corrosion resistance in such environments as air at temperatures up to about 650.degree. C., in most aqueous salt solutions including chlorides, hypochlorites, sulfates, nitrates, and the like, and in many organic chemical environments including most organic acids (Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 20, pp. 369, et seq., 2nd ed. (1969)).
In general, the many grades of commercially pure titanium have better resistance to attack by strong chemicals than do the known alloys of titanium. However, commercially pure titanium has little resistance to corrosive attack by uninhibited, nonoxidizing mineral acids such as hydrochloric, sulfuric, nitric, and phosphoric acids, particularly at elevated temperatures. Although structures fabricated from commercially pure titanium can be employed in these mineral acid environments, if provided with a suitable protective coating usually comprised of a precious metal or oxide thereof, certain titanium alloys have been developed specifically for use in these environments. Typically, the alloys of titanium developed specifically for use in mineral acid environments have been those alloys containing a precious metal as the sole or primary alloying ingredient. Representative of such alloys of titanium are the Grades 7 and 11 specified in ASTM standard B348. In these ASTM grades, palladium is employed as the precious metal alloying ingredient to impart improved corrosion resistance to the titanium.
While various structures have been fabricated from the above described protectively coated commercially pure titanium and alloys of titanium and successfully used in applications where mineral acids were present, the use of such coated or alloyed titanium is not without disadvantages. With respect to both the protectively coated commercially pure titanium and the alloys of titanium, one disadvantage is the high cost of the precious metal material employed to form the coating or the alloy. Further, with regard to the use of protective coatings on commerccally pure titanium, there exists the added necessity of heat treatments at disadvantageously high temperatures to form the coatings and the poor adhesion of the coatings to the titanium.
Thus, a need exists for a titanium possessing good resistance to corrosion when exposed to mineral acid environments and which overcomes or avoids the disadvantages associated with the use of protectively coated, commercially pure titanium and the precious metal containing alloys of titanium. The present invention fulfills such needs.